Multiple grid structure and method of producing the same



April 23, 1940.

e. LIEBMANN 2,197,753 MULTIPLE GRID STRUCTURE AND METHOD OF PRODUCINGTHE Filed June a, 1957 SAME //7 vent-0P.

Patented Apr. 23, 1940 UNITED STATES MULTIPLE GRID STRUCTURE AND METHODOF PRODUCING THE SAME Gerhard Liebmann, Berlin, Germany Application June8, 1937, Serial No. 147,157 In Germany June 16, 1936 1 Claim.

In the patent application Ser. No. 120,774 filed January 15, 1937image-reproducing tubes for television receivers have been describedinwhich a very bright and non-flickering image is pro duced by causingthe cathode ray which moves over the screen area and is controlled inits intensity to create a persistent charge image on a two-dimensionalstructure, termed a multiple grid, which controls in correspondingfashion the electron emission from a two-dimensional cathode.

A multiple grid useful for the purpose in question must comply with anumber of conditions which are diflicult to fulfil. In the first placeit is necessary for the same to consist of a very large number ofconductive elements which are well insulated one against the other.These elements should be at least equal in number to the number of imageelements to be transmitted, or should preferably be even a multiplethereof. Further, the grid must be so dense that it entirely preventsthe passage of electrons from the oathode ray into the image-producingspace, as otherwise the image made visible would be interfered with. Inaddition it of course must be perfectly satisfactory from the point ofview of vacuum technic.

Although it has been found that a mosaic electrode which is deposited onan insulator, such as employed, for example, in television transmissiontubes operating according to the ikonoscope principle, is in certaincircumstances capable of being employed for brief periods as a multiplegrid when the insulating surface is made to face the two-dimensionalcathode, interferences of such extent occur owing to the inavoidablecharging of the insulating front surface that it is necessary to employmultiple grids, in which there is no insulator between the pointscharged by the cathode ray and the cathode of the image-generatingsystem.

The object of the present invention is to provide a suitableconstruction of a multiple grid and a method of producing the same. Inthe drawing Fig. 1 is a side view,

Figs. 2 and 3 are top views showing different stages of manufacture,whilst Fig. 4 shows a structure having a conductive coating applied tothe insulator surfaces.

For this purpose there are cut a sufficiently large number ofcomparatively long aluminium wires, which are coated with a layer ofaluminium. oxide. It is naturally also possible to employ any othermetal wire furnished with a vacuum-resisting and heat-resistinginsulating layer. Owing to their length these wires may be veryconveniently combined into bundles. Over a bundle 1 of wire of thisnature there are now passed seamless rings 8 consisting of asufficiently formed i material, such as steel. These rings have the formof a tube of short cut off sections, and are so narrow that they canjust be fitted comfortably over the bundle of wires. A ring of thisnature having a circular cross-section, is now clamped, together withthe bundle of wires located therein in a four-jaw vice and pressedtogether to get asquare cross-section. In this manner there results areduction in cross-section in the ratio of 4hr. Even if it is consideredthat previously the wires did not completelyfill'out the originalcross-section, the reduction in crosssection caused by the variation inthe form of the holding ring is in any case sufficient to press thewires together very firmly. The plates which are produced by cutting thewires along the edges of the now square frame now possess a sufficientstrength. In addition all gaps between the wires have been completelyclosed, so that the possibility is removed of electrons passing throughfrom the cathode ray space into an image-gen.- erating space. It is alsopossible to fit a large number of rings over a comparatively long bundleof wire, to press at a time one ring, out off the latter, etc.

Multiple grid plates which are produced by embedding wires in a ceramicsubstance possess of necessity the characteristic that the thickness ofthe insulating material is of the same order as I This property may bethe thickness of the wire. utilised for the assembly with the thermalcathode itself, the emission of which is to be controlled by means ofthe multiple grids, by grinding a plate of this nature to be well flaton one surface, providing, by etching or similar methods, for the endsurface of the wires to be situated somewhat below the surface of theplate, and furnishing the resulting perforated insulating surface with aconductive coating which acts as equipotential cathode and at the sametime serves as a support for a material emitting thermal or photoelectric electrons.

After the stated strong etching a multiple grid plate of this kindpossesses the structure of a honeycomb. The advantage of a multiple gridplate produced according to the method as last described resides notonly in the density and strength already referred to, but also in aneasy manufacture and in the possibility of obtaining a very large numberof image points, as very thin' wires can be employed. Thus, for example,it is possible to use Eloxal aluminium wires of 0.1 mm. in diameter,although thin wire of this nature can be coated with aluminium oxideaccording to the Eloxal process only in such short lengths that awinding of coils or a similar method is quite out of the question. Forthe method as last de-- scribed the length of the single wires employedis of no importance in practice.

Multiple grids of the character described may naturally also befurnished with coatings of any desired kind. It is possible, forexample, to activate the individual points photo-electrically or for theemission of secondary electronsor the emission of thermal electrons. Forthis purpose the known methods may be employed, particularly in the caseof the honeycomb construction as last described. For example, aphoto-electric layer can be produced by coating the multiple grid plateby vacuum atomization with a layer of silver interrupted by theinsulating edges, oX-

idizing it and sensitizing it by means of caesium. It is also possibleto make it suitable by means of a barium coating for the emission ofthermal electrons, or to coat it with any other desired layers.

The projecting edge of the insulating material resulting from theetching operation may at the same time serve to ensure the requisitespacing between the metallic wires of the multiple grid and the cathodenet which may then be placed immediately on to the grid, without dangerof short-circuiting. It is only in this way that it is possible tomaintain extremely small spacings. In Fig. 4 there is shown the mannerin which by the etching away of the metal I intermediate structures 6are formed, which maybe furnished with a coating 9 of silver or similarmetal.

A multiple grid plate of this nature may be employed not only in theimage receiving tubes with persistent image referred to at thecommencement, but other possibilities of use are also conceivable bothin the normal amplifying tube art as well as above all in the televisionimage transmission art. For example, a plate of this kind may besuitable to replace the mosaic plate of a transmission tube whichoperates for example in a manner similar to the principle of theikonoscope. In this case it is possible by applying'the grid to a sheetof mica, the rear of which is furnished with a metallic coating, toobtain a capacitative picking-up, it being as well possible to employthe conductive connection of each grid element of the one side with onegrid element of the other side for any other singlesided or double-sidedscanning by means of cathode rays or rays of light.

It has been found desirable to furnish the single wires insulated oneagainst the other and constituting the multiple grid with a commonscreening means, so that the wires of the multiple grid are, as it were,embedded in insulated fashion in a conductive net. This screening meansmay be connected with a desired fixed potential, for example with earth.According to a further feature of the invention, this conductive net isproduced by its coating prior to the assembly,-

each of the insulated wires which form the multiple grid with aconductive outer layer. In the event of Eloxal aluminium Wire beingemployed the production of a conductive layer is not readily possiblewithout reducing the insulation to an inadmissibly low value owing tothe porous nature of the insulating jacket. The pores can be filledhowever by preliminary treatment of the Eloxal wires with aluminiumnitrate or another inorganic insulating material, whereupon a metaldeposit is formed by chemical decomposition in a'manner known per se.This filling of the pores is not required if metal or graphite in solidand very finely distributed form is. taken as a basis. In this case itis sufficient to dip the Eloxal aluminium wire into the conductivepowder. Owing to the fine granular structure of this powder so much ofthe latter remains adhering to the wire, even after shaking, that theconductivity is adequate. This dusting on of the metallic powder isperformed after the bundling but prior to the compression of the wires.Owing to thesubsequent compression the conductive particles are alsoprevented from falling out, so that the desired object is obtained infull extent, as the metal skins surrounding the individual grid wirestouch each other and accordingly establish contact.

It will be understood that in the case that multiple grid plates areproduced from insulated wires, in which each layer has a smooth surface,for example from enamelled wires, the known methods of producing aconductive de-- posit, for example application by vaporization,separation from a solution, spraying on or electrolytic application maybe employed without difiiculty.

A method of producing a photo-electric grid structure consisting of aninsulating plate penetrated by a large number of thin wires insulatedfrom each other, said method consisting in the steps of forming a roundbundle of insulated wires, putting metallic rings upon said bundle,compressing the whole to a smaller cross-section, cutting a plate ofsaid framed bundle, removing the burr from the surface of said plate byetching, and continuing the etching until the end of each wire issubstantially below the surface of the insulating plate.

GERHARD LIEBMANN'.

